Two phase composites of alpha-alumina + 3 mol.% Y2O3-stabilized zirconia (YSZ) with both phases being contiguous were fabricated by sintering powder compacts over a temperature range from 1500 degrees C to 1800 degrees C. The sintered samples were embedded in Na-beta"-alumina powder and thermally treated over a temperature range from 1250 degrees C to 1450 degrees C for up 10 32 hours. This thermal treatment led to the conversion of alpha-alumina + YSZ composites into Na-beta"-alumina + YSZ composites. The mechanism of conversion involves a coupled transport of Na+ through the Na-beta"-alumina and of O2- through the YSZ. The kinetics of conversion was investigated by measuring the conversion thickness as a function of time. A model for the conversion kinetics is developed which involves two interface steps (at the gas/solid interface and at the alpha-alumina + YSZ/Na-beta"-alumina YSZ reaction front) and a coupled transport of Na+ through Na-beta"-alumina and of O2- through YSZ. The data are analyzed using the model developed. At small thicknesses, the kinetics tends to be interface-controlled. At large thicknesses, the kinetics approaches diffusion control. (C) 2013 The Electrochemical Society. All rights reserved.